The present invention relates generally to a crystal pulling mechanism for the growth of semiconductor crystals. More particularly, the present invention relates to a procedure for correcting speed deviation between actual and nominal pull speed during growth of such crystals.
Most processes for fabricating semiconductor electronic components are based on single crystal silicon. Conventionally, the Czochralski process is implemented by a crystal pulling machine to produce an ingot of single crystal silicon. The Czochralski or CZ process involves melting highly pure silicon or polycrystalline silicon in a crucible located in a specifically designed furnace contained in part by a heat shield. After the silicon in the crucible is melted, a crystal lifting mechanism lowers a seed crystal into contact with the silicon melt. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt. The resulting crystal is substantially free of defects and therefore suitable for manufacturing modern semiconductor devices such as integrated circuits. Other semiconductors such as gallium arsenide, indium phosphide, etc. may be processed in similar manner.
CZ production typically has very strict process control requirements to ensure quality of and uniformity of the resulting crystal. Especially important are process parameters that affect intrinsic material properties or affect process stability and process yield. Pulling speed accuracy is one of the most important parameters among these process parameters.
After formation of a crystal neck or narrow-diameter portion, the conventional CZ process enlarges the diameter of the growing crystal. This is done under automatic process control by adjusting the pulling speed or the temperature of the melt in order to maintain a desired diameter. The position of the crucible is adjusted to keep the melt level constant relative to the crystal. By controlling the pulling speed and the melt temperature, and by decreasing the melt level, the main body of the crystal ingot grows with an approximately constant diameter. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable or shaft along with the seed and the crystal in an opposite direction.
The pulling speed is a closely controlled parameter. A control unit such as a personal computer or programmable logic controller is controlled to respond to input signals by providing control signals to control electromechanical devices that pull the crystal from the melt. These include one or more electric motors.
Pull speed accuracy is determined in part by how accurately a speed signal from the control unit is translated into actual mechanical crystal pulling speed. Between the control signal and the crystal are several components that can introduce errors. It is well known that the servo electronics that drives the electric motor are susceptible to drift and therefore must be regularly calibrated. However, the mechanical parts, particularly the gears that reduce the motor speed down to crystal pulling speed, are also a significant source of speed errors. For instance, almost all cable type CZ pullers employ a design that uses a worm drive at the final stage of speed reduction.
A worm drive is a gear arrangement in which a worm meshes with a worm gear, also called a worm wheel. The worm is a gear in the form of a screw. The worm is rotated about a first axis, producing rotation of the worm gear about a second, generally perpendicular, axis. The worm drive can reduce rotational speed and transmit higher torque to the output shaft. Also, worm gears are well suited for lifting heavy loads because they are self holding and allow large speed reduction ratios. These are some of the reasons they have been widely adopted in CZ applications.
However, worm gears by design can never produce a truly constant speed reduction, because of the periodically changing contact point between sprocket teeth of the worm gear and worm thread. In the best case, when high quality or new gears are used, the result is a tooth-by-tooth periodicity in the speed reduction ratio on the order of fractions of a percent. In the worst case, when inexpensive or heavily worn gears are used, the result can be a wildly varying speed reduction by several percent.
Unfortunately, it is a reality in CZ mass production that mechanical parts do wear out. It is also a reality that many pullers do not use high quality speed reducer gears. The gears may be of low quality by having, for example, only 30 sprocket teeth per turn. Also, wear may be enhanced in some applications because the worm gear is regularly overloaded every time the crystal reaches its final weight. This results in significant tooth wear within the gear's normal life span. Tooth wear is manifested as thinning or distortion of the teeth and increasing of the spaces between the teeth at tooth surfaces that engage the worm. The result is heavy tooth-by-tooth variation of the gear's actual output speed. In addition, a significant systemic variation of speed reduction occurs over the course of one turn, because the teeth wear out unevenly due to the linearly increasing load as the crystal is pulled up.
FIG. 1 illustrates gear tooth wear on a gear having 30 teeth per turn. FIG. 1 shows output speed correlation (or percent of calibration) relative to nominal speed as a function of gear tooth for three different test runs, labeled #1, #2, and #3. The gear under test is driven by a servo motor and output speed is measured relative to a calibrated value. The gear wear creates an offset relative to the calibrated value.
As can be seen in FIG. 1, the result is heavy tooth-by-tooth variation of the gear's actual output speed. The speed variation can be quite sudden from tooth to tooth. In addition, FIG. 1 shows a significant systemic variation of speed reduction over the course of one turn, because the teeth wear out unevenly due to the linearly increasing load as the crystal is pulled up.
FIG. 2 illustrates speed correlation for a worm gear as a function of gear tooth for a new, high quality worm gear having 60 teeth per turn. FIG. 2 illustrates that even quality worm gears with high load reserve and little wear produce a varying actual output speed. Thus, even a brand new, high quality worm gear will exhibit output speed variations caused by the periodically changing contact point between sprocket teeth of the worm gear and the worm thread.
While the problem of output speed variation has been described herein in connection with worm gear wear and quality, it should be noted that any number of other mechanical or electrical variations can produce similar output speed variation of the type described. There is a need for an improved crystal pulling system which can correct such output speed variation.